To define the surface ultrastructure of leukocytes and clarify the physiology of normal and leukemic cells; also, to determine the origin and nature of granulocyte inclusions as indices of development, function, and fate. Aliquots of white cells, concentrated from the anticoagulated blood of normal human donors as buffy coats and by the Ficoll-Hypaque procedure, were fixed in suspension for SEM, and parallel aliquots were incubated on nylon fiber columns; non-adherent and adherent leukocyte populations were collected from the columns and fixed in suspension for scanning electron microscopy (SEM). Leukocytes fixed in suspension were deposited, stained, and washed on Nuclepore filters, trapped in a thin film of water, quick frozen, and freeze-dried for SEM with 90% yields. Individual cells examined by SEM were relocated by light microscopy (LM) and additional specimens have been prepared for transmission electron microscopy (TEM). Chemotactic studies were carried out with guinea pig peritoneal macrophages in modified Boyden chambers equipped with Nuclepore filters (3.0 um pore size). The response of leukocytes on one side of the filter to the presence of control phosphate-buffered salt solution (PBS) or to the chemotactic factors C5A and LDCF on the other side was studied by fixing the filters at different times (30, 60, 90, and 120 minutes). Light microscopy was done on some of the same scanned cells as above.